What are the current standards and guidelines for highway stormwater runoff quality and quantity?

1 Regulatory Framework

The main federal law that regulates highway stormwater runoff is the Clean Water Act (CWA), which aims to protect and restore the nation's waters from pollution. Under the CWA, highway projects that disturb one acre or more of land are required to obtain a National Pollutant Discharge Elimination System (NPDES) permit from the Environmental Protection Agency (EPA) or an authorized state agency. The NPDES permit sets limits and conditions for the discharge of stormwater runoff from the construction and operation phases of highway projects. The permit also requires the development and implementation of a Stormwater Pollution Prevention Plan (SWPPP), which outlines the best management practices (BMPs) to control and treat stormwater runoff on site.

2 Design Criteria

The design criteria for highway stormwater runoff vary by state, region, and project type, but typically involve hydrologic analysis, hydraulic design, and water quality design. Hydrologic analysis involves estimating the volume, peak flow, frequency, and duration of stormwater runoff using methods such as the Rational Method or the Soil Conservation Service Curve Number Method. Hydraulic design involves designing drainage systems to convey the stormwater runoff safely and efficiently using methods such as the Manning Equation or the Energy Equation. Water quality design involves selecting and sizing BMPs to reduce pollutant load and improve water quality using methods such as the Simple Method or the EPA Stormwater Management Model.

3 Performance Standards

Performance standards for highway stormwater runoff are the goals that a highway project should achieve in terms of runoff quality and quantity. These standards are based on regulatory requirements, site characteristics, and environmental objectives. Examples of performance standards include peak flow reduction to prevent or minimize flooding and erosion, runoff volume reduction to enhance groundwater recharge and reduce pollutant load, and pollutant load reduction to protect and improve downstream water body quality. These standards are measured by the percentage or amount of reduction in the pre-development to post-development condition.

4 Maintenance and Inspection

The maintenance and inspection of highway stormwater runoff facilities are essential to ensure their proper function and performance over time. Activities such as cleaning and clearing, repairing and replacing, and monitoring and testing must be carried out regularly. Cleaning and clearing involves removing trash, debris, sediment, and vegetation from the drainage systems and BMPs to prevent clogging. Repairing and replacing includes fixing or replacing damaged or worn-out components of the drainage systems and BMPs to prevent leakage, collapse, or failure. Lastly, monitoring and testing measures and evaluates the flow, volume, and quality of stormwater runoff from the highway project to verify compliance with the NPDES permit and the performance standards.

5 Resources and Tools

Highway designers can find an abundance of resources and tools to learn more about the current standards and guidelines for highway stormwater runoff quality and quantity. For example, the Federal Highway Administration (FHWA) provides guidance, manuals, reports, webinars, and training on topics like hydrology, hydraulics, water quality, BMPs, performance evaluation, and maintenance. The Environmental Protection Agency (EPA) offers regulations, policies, technical assistance, and tools like the NPDES permit program, the SWPPP template, the BMP database, and the Stormwater Management Model. Additionally, the American Society of Civil Engineers (ASCE) provides standards, publications, conferences, and courses such as ASCE/EWRI 45-16 Standard Guidelines for Stormwater Quality Compliance and ASCE/EWRI 60-10 Standard Guidelines for Stormwater Management Systems Operation and Maintenance.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?